Herschelian telescope

A Herschelian telescope is a reflecting telescope, of a design conceived
by William Herschel and used in his giant 48-inch
instrument, in which the primary mirror is tilted so that light is focused
near one side of the open end of the tube. The eyepiece then picks up this
light directly, avoiding light loss from reflection by a secondary mirror.
The drawback is astigmatism, unless
the focal ratio is large.